1. Field of Invention
The invention relates generally to a method and system for providing tunable dispersion compensation.
2. Description of Related Art
Dispersion is a known phenomenon in optical networks that causes a broadening of optical pulses along the length of the fiber. One type of dispersion relevant to the present invention is chromatic dispersion (also referred to as “material dispersion” or “intramodal dispersion”), caused by a differential delay of various wavelengths of light in a waveguide material.
The spectrum associated with a transmitted optical signal is subject to modulation-induced broadening, which increases linearly with bit rate. At the same time, the bit period decreases linearly with increasing bitrate. These two effects combined produce a quadratic scaling of signal intolerance to dispersion with increasing bitrate. Accordingly, for example, a 10 Gbps signal is 16 times less tolerant to dispersion than 2.5 Gbps signal, but only 4 times the bit rate.
Dispersion accumulates linearly with propagation distance in the fiber. Without compensation, typical propagation distances in standard single-mode fiber (e.g., SMF-28 or equivalent) are ˜1000 km at 2.5 Gbps, 60 km at 10 Gbps, and only ˜4 km at 40 Gbps. Clearly, some form of dispersion compensation is required to obtain substantial propagation distances at bit rates of 10 Gbps and above.
Moreover, fiber-optic system transport capacity has been increasing through combining multiple, separately modulated optical carriers at distinct wavelengths onto a single fiber. This technique is known as wavelength-division multiplexing (WDM). Due to WDM, it is preferable that dispersion compensation be performed for multiple wavelengths using a common device.
Several methods have been proposed to compensate for dispersion, including fiber Bragg gratings, optical all-pass interference filters, and dispersion compensating fiber. Dispersion compensating fiber (DCF) has found widespread practical acceptance and deployment due to its numerous advantages. Such advantages include relatively low loss as well as cost and ability to provide dispersion compensation across a broad range of wavelengths.
Fiber carrying optical signals in the 1550 nm communication window often has a positive dispersion, which can be offset or compensated with DCF having an associated negative dispersion, i.e. a negative dispersion coefficient. The length of DCF is selected so that the product of dispersion coefficient and length of DCF equals, in magnitude, the product of dispersion coefficient and length of transmission fiber. In which case, the DCF fully compensates the dispersion incurred by the transmitted optical signals. It may also be desirable to provide dispersion compensation that does not entirely compensate for dispersion in the transmission fiber, which may be useful for addressing certain nonlinear effects of signal propagation.
As a practical matter, the dispersion coefficient associated with DCF is typically greater in magnitude than the dispersion coefficient of the transmission fiber for which it compensates. Accordingly, the length of DCF used in conventional transmission systems is often shorter than the transmission fiber length.
A drawback to the DCF compensation schemes described above is that DCF lengths must be altered to if the length of transmission fiber is changed in order to provide the same amount of compensation. Moreover, if the dispersion associated with the transmission fiber and/or the DCF changes (e.g., due to environmental factors such as temperature), the DCF no longer provides full compensation. Thus, there is a need in the art for a tunable dispersion compensation system in which the dispersion associated with DCF can be altered.